We are building complex systems all the time, and it's actually scary how many defenses against failure are built into them. These defenses can be as simple as checking return value of function, or more complex with fallbacks and alternative implementations. They aren't scary because they are there; they are scare when you think that if even one of those defenses is missing, things go bad pretty quickly.

Currently humans are still superior in defending these systems. They make workarounds and processes that avoid potential failures. It might be really interesting to apply machine learning in these situations, trying to find out the sets of actions that lead to failures.

But meanwhile, we have to learn from our systems by ourselves, so try to avoid hunting that one root cause.

keskiviikko 20. syyskuuta 2017

AWS Cloudformation has multiple different commands in aws cli, like "create-stack", "update-stack" and "deploy". Each of these have their good and bad sides. For multiple reasons, we've decided to use "deploy". But the problem then becomes tagging. "Create-stack" and "update-stack" both have support for giving tags which are then propagated to all supported resources, but deploy does not have it. To make things worse, some Cloudformation types does not support tags as their properties, but they seem to get tags from Cloudformation stack if tags are there.

Now we do after deploy "aws cloudformation update-stack --stack-name <some> --tags ...". This becomes quite easy with some scripting when you have jq!

As update-stack wants to have all parameters with "UsePreviousValue=true", use some jq to generate necessary parameters. Then we take existing Parameters we've used for tagging and generate tags from that.

Well, actually "quite easy" is a lie, as I had some problems in understanding right syntax to replace key in JSON array with jq.

tiistai 19. syyskuuta 2017

For a while, I've been strugling to get dillon's cron working properly in Docker container. The problem has been that when the ENTRYPOINT was anything else than in shell form, I got 'setpgid: Operation not permitted'.

So, this worked:

ENTRYPOINT /usr/sbin/crond -f

None of these seemed to work:

ENTRYPOINT ["/usr/sbin/crond", "-f"]

Or

ENTRYPOINT ["./entrypoint.sh"]
CMD ["/usr/sbin/crond", "-f"]

As both would give

setpgid: Operation not permitted

But using shell form has been enough, for now. Now as I finally needed to have entrypoint for doing some preparation work, something had to be done.

"su -c" to the rescue.

ENTRYPOINT ["./entrypoint.sh"]
CMD ["su", "-c", "/usr/sbin/crond -f"]

Seems to be working perfectly.

perjantai 30. syyskuuta 2016

I've got a small gluster of six Raspberry Pies, and I wanted to update them all. I'm just too lazy to update every SD card one by one, so I wondered if it is possible to do the update without removing SD cards from Raspberries. And it is!

Then you can boot up the Raspberry and start the NOOBS recovery by pressing shift -key during start up. But I'm too lazy to do even that. Luckily it is possible to make the NOOBS automatically start recovery and install new OS.

The behaviour of NOOBS can be controlled with commandline options. These options are defined in file called "recovery.cmdline" in the root of /dev/mmcblk0p1. The default contents of the file are following:

To make the installer start by default, you have to add "runinstaller" option. This only starts the installer, but it will need user input to continue. Another option, silentinstall, will tell the installer to go forth and install OS. Just make sure that there is only one OS in os/ -directory, and if it has more that one flavour, edit it's flavours.json file (details in https://github.com/raspberrypi/noobs#how-to-automatically-install-an-os).

After installation, the installer does remove the "runinstaller" -option from recovery.cmdline so it does not reinstall on every boot. The "silentinstall" option remains, though.

So when everything is in place, at next reboot, there's a new version of noobs, and it will install the OS automatically. Just remember, that everything on Raspberry will be wiped!

Here's a ansible playbook that does everything. It will take quite a while to complete, as the NOOBS image file is pretty big and takes a while to download and transfer to hosts. Reason why I'm downloading NOOBS to local machine is that I'm running this playbook with six Raspberries and it should be faster to download NOOBS once to local machine and the transfer it to Raspberries instead if downloading it on every Raspberry

tiistai 5. heinäkuuta 2016

At work, we wanted send an email for every change that was made in a project. By default, Jenkins likes to collate changes into as few builds as possible, and normally sends an email per build.

The solution seemed to be usage of Jenkins Pipeline. Jenkins Pipeline enables creation and execution of jobs "on the fly" as needed.

First problem was to get access to ChangeLogSet. There is some preset variables in Jenkinsfile, but I could not find documentation for them. After some googling, Stack Overflow came to rescue.

def changes = currentBuild.rawBuild.changeSets

But when this was executed, Jenkins complained

org.jenkinsci.plugins.scriptsecurity.sandbox.RejectedAccessException: Scripts not permitted to use method org.jenkinsci.plugins.workflow.support.steps.build.RunWrapper getRawBuild

There's a "In-process Script Approval" -tool in Jenkins, where you can allow usage of these methods.

After that was solved, next problem was with serialization. As the actual job execution is transferred to different node, every non-serializable object caused an exception. To prevent this, I had to null objects in proper places. This then prevented running jobs in loop, as the variables in loops needed to be nulled before job execution. So I had to collect jobs into map, and after every job was defined, null everything and use "parallel" -task to execute jobs.